In the art of electroerosion with wire type electrode, which is commonly called "wire-cut EDM (electrical discharge machining)" or "travelling-wire EDM", a wire or like elongated body, which is continually fed or caused to travel through a machining zone in which a workpiece is positioned upon a worktable, is supported under tension between a pair of guides disposed at opposite sides with respect to the workpiece while a series of electrical pulses are applied between the workpiece and the wire which constitutes a machining electrode to effect time-spaced electrical discharges between them. The machining zone is flushed with machining fluid which is typically a distilled water and material is removed from the workpiece by the electroerosion action. As material removal or machining proceeds, the worktable carrying the workpiece is displaced transversely to the direction of travel of the workpiece by control signals delivered from a control unit that is typically a numerical controller in which a predetermined cutting path is memorized. In usual applications, the wire electrode travel extends perpendicularly to the direction of such displacement of the workpiece relative to it so that a straight cut with a predetermined pattern is produced in the workpiece.
It is sometimes desirable to obtain a tapered cut in a workpiece, for example, when the latter is to be a mold or die with a tapered edge. In one conventional process, one of the guides which support the wire electrode at opposite sides with respect to the workpiece is caused to continually move along a circle having a radius predetermined in conjunction with the angle of a taper desired. In another known process, the wire electrode is caused to travel under numerical control in a path normal to the direction of advance of machining along a predetermined circular orbit with a predetermined slope.
Both of these processes have disadvantages. The first technique entails excessive amount of material removal because of a frustoconical cutting that ensues and is hence time-consuming and ineffective. Further, corner portions become rounded and consequently inaccurate. Besides, this technique is only applicable with practicability to thin workpieces and small tapers. In the latter process as well, corner portions become rounded. Thus, during the time in which the rotary table which carries one of the wire-supporting guides along the predetermined circular orbit is rotated in order to cause the wire electrode to be oriented normal to the direction of advance in the next machining block or section, the worktable which carries the workpiece for contour-machining under numerical control must be kept stationary. This brings about over-cutting of the workpiece portion which is located on the side of the fixed electrode-supporting guide with the result of deterioration of machining accuracy.